eMarketer anticipates that mobile ad spending will hit $4 billion by year's end, up 180% from 2011 — a substantial revision of the forecast the firm released just two months ago, which said that mobile ad spending is expected to increase 80% to $2.61 billion for the year. That figure includes a variety of ad products designed to run on phone and tablet devices, including search, display and messaging-based ads.

Facebook alone is expected to account for $339 million of U.S. mobile ad dollars in 2012 with an 18.4% share of the mobile ad display market, according to eMarketer. Indeed, Facebook's mobile ad unit grew from 0% to 14% of total ad revenue in just six months, and is now generating more than $3 million per day.

Twitter is a smaller force: eMarketer estimates that Twitter will generate $134.9 million in mobile ad revenue by year's end, accounting for 3.5% of total U.S. mobile ad spending and 7.3% of mobile display ad spending. Both Twitter and Facebook are expected to top Apple, which owns an estimated share of 6.7% of the mobile display market via its iAds product.

Google is still dominant in the overall mobile ad space however, with an estimated market share of 56.6%. Its share of the $1.99 billion mobile search ad market is almost entirely dominant at an estimated 93.3%.

2013 is expected to be an even stronger year for mobile advertising. eMarketer predicts that U.S. mobile ad spending will reach $7.19 billion next year, and more than $20 billion by 2016. Still, mobile only represents a tiny fraction — just 2.4% — of total ad spending in the U.S. By 2016, that share is expected to increase to 11%, overtaking radio but not print.

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